The present invention relates to the use of light modulation in the field of optical switches for optical networks.
Modern communication systems frequently use light carriers because of the large bandwidth inherent in light signals. In order to use light as a carrier, however, it is necessary to build switches that can make the necessary connections between the source and the destination of the signal. Traditionally, these switches involve modulation and demodulation to convert the optical signals into electrical signals and then convert the electrical signals back to optical signals. Such traditional electronic interconnection techniques have several physical limitations, such as poor synchronization and low bandwidth, which cannot support the interconnection density, speed, and signal bandwidth of an optical network. To overcome these problems, optical interconnection techniques have been developed. (Such techniques are detailed, for example, in J. W. Goodman et al., Proc. IEEE, 72, 850 (1984) and A. Husain, SPIE 466, 24, (1984)).
There are several techniques for optical interconnections. One area of active development in optical interconnection techniques at present is the use of reconfigurable liquid crystal devices such as spatial light modulators (SLMs) in optical switches. An SLM can be generally characterized as a reconfigurable optical element (e.g., a reconfigurable liquid crystal device). SLMs are available in both electrically-addressable (EASLM) and light-addressable (LASLM) configurations. Often each pixel in a reconfigurable liquid crystal device acts as an independent rotatable waveplate, such that an applied voltage rotates the birefringent axes. Owing to this property, liquid crystal light modulators may be used as holographic optical elements (HOEs), which are capable of producing computer generated holograms (CGHs). Such computer generated holograms have particular applications to free space diffraction-based optical switching. For example, SLM-based holographic optical elements (HOEs) can be used to connect light either from a single source simultaneously to a number of destinations or conversely from a plurality of sources to a single destination. This is described in U.S. Pat. No. 5,768,242, issued to Juday and assigned to the same assignee hereof. This patent is incorporated herein by reference.
For a cross bar switch with a dimension of Nin and Nout at input and output, respectively, the number of potential connection points is Ninxc3x97Nout. Though they need not all be realized for the device to be useful, the number of possible connection patterns is exponential in that product. In the matrix of connection patterns, each connection can independently be on or off, so the number of connection patterns is 2(Ninxc3x97Nout). In a simple 2xc3x972 array, for example, there are 2(2xc3x972)=16 potential interconnection patterns. If inputs are enumerated as A and B and outputs as 1 and 2, these patterns are: (none), A1, B1, A2, B2, A1andB1, A1andA2, A1andB2, A2andB1, B1andB2, A2andB2, A1andA2andB1, A1andB1andB2, A1andA2andB2, A2andB1andB2, A1andA2andB1andB2. The number of possible interconnection patterns becomes even more complex with increasing numbers of input and output optic fibers. In order to handle such complex interconnections, flexible and versatile switches are required. The SLM-based optical switches are particularly well-suited for this purpose.
Most SLM-based optical switches are shift invariant, i.e., when the input and the output shift in a corresponding amount but the output is not otherwise altered. Shift invariance of a holographic connection dictates that each of the input sources will be diffracted into the same number of output spots because the patterns of locations to which the output spots are directed are translated in accordance with how the input sources are translated with respect to each other. Thus, it becomes important that in selecting patterns of input locations, output locations, and locations of holographically created spots to make optical connections, all three of these items be simultaneously considered.
Traditionally, SLM-based optical switches have their input and output optic fibers arranged in a regularly spaced rectilinear geometry. This geometry exacerbates the cross-talk problems resulting from coherent buildup of diffractive sidelobes. The rectilinear geometry also makes poor use of the real estate in the area, when such diffractive sidelobe-related cross-talk can be avoided by moving a receptive area off to the side of a rectilinear array of spots. Therefore, it is desirable to have a method to optimize the locations of the output optical fibers in order to minimize cross-talk.
In order to optimize the arrangement of the input and output optic fibers in the optical switches, it is necessary that the holograms be accurately computed. Prior art SLMs are often modeled as being some ideal: amplitude variation is used to block light and phase is used to retard, and thus redirect, it. These computations of holograms have been done under the assumption that the behavior is purely phase variation or purely amplitude variation. In reality, amplitude and phase often co-vary. Incorrect modeling of the behavior of the light control causes less than optimal results in the face of realistic physical devices. U.S. Pat. No. 5,768,242 discloses a process which fully accommodates the actual behaviors of SLMs; this process acknowledges that phase (retardation) and amplitude of transmission co-vary. With this process, it is possible to accurately model the holograms for optimizing the geometry in an optical switch in order to minimize cross-talk.
Most observed actions of SLMs depend on the polarization states of the incoming light and, if there is a polarizer (whether intentionally placed or implicit in the optics), the outgoing optics. However, there is no guarantee of the polarization state of incoming light from many kinds of fiber optic lines. As a result, modulation by SLMs tends to be highly variable. This is especially problematic with a full crossbar switch, because the light beams from several fiber sources are simultaneously involved, and their polarizations may be random with respect to each other, unless these polarizations are specifically controlled, for example, by the use of expensive polarization-maintaining fiber. Therefore, it is desirable that an optical switch be capable of handling incoming light with arbitrary polarization states to produce effects that are invariant to the incoming polarization states.
One aspect of the invention relates to optical switches for connecting a light source to a light receiver. One embodiment of the invention is a polarization-independent optical switch for switching at least one incoming beam from at least one input source to at least one output drain. The optical switch comprises a polarizing beam splitter to split each of the at least one incoming beam into a first input beam and a second input beam, wherein the first input beam and the second input beam are independently polarized; a wave plate optically coupled to the second input beam for modifying the polarization of the second input beam to generate a modified second input beam; a beam combiner optically coupled to the first input beam and the modified second input beam, wherein the beam combiner refracts and transmits the first input beam and the modified second input beam to produce a combined beam; and a controllable spatial light modulator optically coupled to the combined beam, wherein the combined beam is transmitted by the controllable spatial light modulator that effects a hologram. This optical structure is referred to as a xe2x80x9cpolarization rectifierxe2x80x9d. It delivers light from an arbitrary input polarization state into a given output polarization state. Further, it does so without extinguishing the light in any input polarization state; this feature discriminates it from an optical analyzer.
Another aspect of the invention relates to an optical switch system, comprising a plurality of input optical fibers, a controllable spatial light modulator-based optical switch optically coupled to the input optical fibers, and a plurality of output optical fibers optically coupled to the controllable spatial light modulator-based optical switch, wherein the output optical fibers are not arranged in a rectilinear geometry.
Another aspect of the invention relates to a method for switching an optical network connection from a plurality of incoming optical fibers to a plurality of output optical fibers. The method comprises passing an incoming beam from the input optical fibers through a polarization rectifier to produce a beam in a particular polarization state (xe2x80x9cparticularly polarized beamxe2x80x9d), and passing the particularly polarized beam through a controllable spatial light modulator, wherein the particularly polarized beam is diffracted and transmitted by the controllable spatial light modulator to form a pattern of light spots with at least one light spot landing on at least one output optical fibers.
Yet another aspect of the invention relates to a method for manufacturing a controllable spatial light modulator-based optical switch. The method comprises assembling an optical switch between a plurality of input optical sources and a plurality of output optical drains, wherein the optical switch comprises a controllable spatial light modulator (and possibly a polarization rectifying optics), and tuning the controllable spatial light modulator, wherein a hologram effected in the modulator causes a light beam from the incoming optical fibers to have light spots landing on the outgoing optical fibers.
It should be appreciated that the term xe2x80x98optical fibersxe2x80x99 are used herein to refer generally to the input optical sources and the output optical drains. The input optical sources may include light-emitting diodes, laser diodes, or other suitable sources of input light; similarly, the output optical drains may include detectors of light or light guides other than fibers at the output locations. In addition, a spatial light modulator is often described as being a xe2x80x98liquid crystalxe2x80x99 modulator. However, where polarization is important to the modulator""s operation, any birefringent modulator may be used, not just one that uses liquid crystal as the active medium. When the polarization state is not of primary importance to the function of the modulator (e.g. where the modulator functions by moving small mirrors), the modulator may use other devices.